Quality inspection of leather using novel planar sensor : a thesis submitted in fulfilment of the requirements for the degree of Master of Engineering (Research), School of Engineering and Advanced Technology, Massey University, Turitea campus, Palmerston

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(1)Copyright is owned by the Author of the thesis. Permission is given for a copy to be downloaded by an individual for the purpose of research and private study only. The thesis may not be reproduced elsewhere without the permission of the Author..

(2) QUALITY INSPECTION OF LEATHER USING NOVEL PLANAR SENSOR. A Thesis Submitted in Fulfilment of the Requirements for the Degree of Master of Engineering (Research). VISHNU MOHAN KASTURI. School of Engineering and Advanced Technology, Massey University, Turitea Campus, Palmerston North, September 2008.

(3) “This dissertation is dedicated to my Family”.

(4) ABSTRACT. Value of leather produced from sheep is determined by its quality and looseness is one of the quality attributes that determines the value of the leather. As of now, looseness in sheep skin can be determined only after the tanning process is done and it is a long and expensive process to treat the looseness in skins after the tanning process. An interdigital sensor based sensing system has been developed which works on the principle of sensing technique based on interaction of electric field with the materials under test. Finite element software has been used for analysis and design of sensors. It has been reported that a good correlation was found between the actual looseness values and calculated looseness values.. i.

(5) Acknowledgements Firstly I would like to thank, Dr. Subhas Mukhopadhyay for giving me an opportunity to do my masters under his supervision. His Wisdom, knowledge and continuous support always inspired and motivated me and I am indebted for his technical, financial and emotional support. I will always be grateful for providing opportunities to publish and present my work at various conferences. I would like to thank Mr. G. Sen gupta for his help regarding programming microcontroller. I would also like to thank Dr. Tim Alsop (LASRA) for his valuable inputs about the sheep skins and Leather and Shoe Research Association (LASRA) for providing the samples for experimentation. On a personal level I would like to thank all my friends especially Ch. Naga Srikanth and Barnendar who helped me emotionally and financially to reach my goals. I would also like to thank my brother Madhan Mohan and his wife Swarna for just being a phone call away and most importantly my parents Mr. Krishna Gopal and Mrs. Shailaja for their unconditional love, support and all the sacrifices they made to get me to this position. Finally I would like to thank all technical and non-technical staff at SEAT for helping me through various stages.. ii.

(6) PUBLICATIONS Below are the publications in conjunction with the authors Masters Candidacy:. Conference Publications. 1. V. Kasturi, S.C. Mukhopadhyay, G. Sengupta, “Embedded Microcontroller Aided Planar Interdigital Sensor Based property Estimation of Sheep Skin”, 14th Electronics New Zealand Conference (ENZCon 2007), Victoria University of Wellington, Wellington, New Zealand, 12 – 13 November, 2007. 2.. V. Kasturi, S.C. Mukhopadhyay, G. Sengupta, “Interdigital Sensors: A Review of their Applications”, 2nd International Conference on Sensing Technology ( ICST) Massey University, Palmerston North, New Zealand, November 26-28, 2007.. 3.. V. Kasturi, S.C. Mukhopadhyay, Y. M. Huang, “A Novel Bio-sensor for Noninvasive Sensing of Sheep Skin”, 4th Asia Pacific Conference on Transducers and Micro/Nano Technologies (APCOT 2008), National Cheng-Kung University, Tainan, Taiwan, pp. 251 – 254, 22 – 25 June, 2008.. 4.. A. R. Mohd Syaifudin, S.C.Mukhopadhyay and V. Kasturi, “Smart Sensing System for Health and Environmental”, Digital Signal Processing Creative Design Contest (DSP 2008), Southern Taiwan University, 29 November, 2008.. iii.

(7) 5.. V. Kasturi, S.C. Mukhopadhyay, “Planar Interdigital Sensors Based Looseness Estimation of Leather “, 3rd International conference on sensing technology, National Cheng-Kung University, Tainan, Taiwan, pp. 462 – 466, Dec 1 – Dec 3, 2008.. Journal Publications 1.. V. Kasturi, S.C. Mukhopadhyay, T. Allsop, S. Deb Choudhury, G. E. Norris, “Assessment of pelt quality in leather making using a novel non-invasive sensing approach”, Journal of Biochemical and Biophysical methods, Volume 70, issue 6, pages 809 – 815, 24 April, 2008.. Textbook Publications Work is published in the Sensors book by Springer. 1.. S. C. Mukhopadhyay, Y. M. Huang, “Estimation of Property of Sheep skin to. Modify the Tanning Process”, Sensors: Advancements in Modeling, Design Issues, Fabrication and Practical Applications - Springer, pp. 91 – 112, July 2008.. Presentations. 1.. Participated in IEEE pacific zone seminar, December 2007.. 2.. Presented my research work at IEEE Postgraduate student presentation day,. August 2008.. iv.

(8) Contents ABSTRACT. i. ACKNOWLEDGEMENT. ii. PUBLICATIONS. iii. CONTENTS. iv. LIST OF FIGURES. viii. LIST OF TABLES. xv. CHAPTER 1 INTRODUCTION. 1. 1.1 Introduction. 1. 1.2 Non-Destructive Evaluation. 1. 1.3 Sensors. 4. 1.4 Objective of research. 8. 1.5 research on skin property estimation. 10. 1.6 Organization of Thesis. 11. CHAPTER 2 LEATHER: EVALUATION OF QUALITY. 12. 2.1 Introduction. 12. 2.2 Structure of sheep skin. 12. 2.3 Looseness. 16. 2.4 Factors affecting looseness. 17. 2.5 Processing of Sheep skin. 21. 2.6 Tanning in ancient history. 22. v.

(9) 2.7 Modern methods of Tanning. 22. 2.8 Types of Leather. 26. CHAPTER 3 INTERDIGITAL SENSORS. 28. 3.1 Introduction. 28. 3.2 Operating principle of Interdigital sensors. 28. 3.3 Applications of Interdigital sensors. 34. CHAPTER 4 EXPERIMENTAL SET-UP AND ANALYSIS OF. 38. SENSORS 4.1 Introduction. 38. 4.2 Design of Interdigital sensors. 38. 4.3 Finite element modeling of Interdigital sensors. 46. 4.4 Preliminary experiments. 56. 4.5 Experimental set-up. 62. 4.6 Conclusion. 64. CHAPTER 5 EXPERIMENTAL PROCEDURE AND RESULTS. 65. 5.1 Experimental procedure. 65. 5.2 Observations for sheep skins before Tanning. 68. 5.3 Looseness values for sheep skins. 75. 5.4 Effect of thickness of sheep skin on sensor voltage. 89. 5.5 Observations for sheep skins after Tanning. 95. 5.6 Calculation of looseness in sheep skin. 109. 5.7 Conclusion. 114. vi.

(10) CHAPTER 6 DATA ACQUISITION SYSTEM. 115. 6.1 Introduction. 115. 6.2 Data acquisition system. 115. 6.3 Experimental results. 116. 6.4 Conclusion. 117. CHAPTER 7 CONCLUSION AND FUTURE WORKS. 118. 7.1 Conclusions. 118. 7.2 Recommendations and future work. 120. 121. CHAPTER 8 REFERENCES. vii.

(11) LIST OF FIGURES Figure 1.4.1 Cross section of loose leather with extra spaces between the fibres. 8. Figure 1.4.2 Cross Section of tight leather with less space between the fibres. 9. Figure 2.2.1 Cross section of sheep skin. 13. Figure 2.2.2 Leather samples with fat cells and looseness shown. 15. Figure 2.2.3 Looseness scale determined by LASRA. 16. Figure 3.2.1 Operating principle of an Interdigital sensor. 29. Figure 3.2.2 Interdigital sensor structure. 30. Figure 3.2.3 Electric field formed between two electrodes for different pitch. 31. Figure 3.2.4 Penetration depths for varying spatial lengths between the electrodes. 31. Figure 3.2.5(a) Sensing the material density. 32. Figure 3.2.5(b) Measure the distance between sensor and the material. 32. Figure 3.2.5(c) Track the structure of the material under test. 33. Figure 3.2.5(d) Sensing the moisture. 33. Figure 4.2.1 Image of sensor 1. 38. Figure 4.2.2 Design configuration of sensor 1. 39. Figure 4.2.3 Image of sensor 2. 40. Figure 4.2.4 Design configuration of sensor 2. 40. Figure 4.2.5 Image of sensor 3. 41. Figure 4.2.6 Design configuration of sensor 3. 42. Figure 4.2.5 Image of sensor 4. 43. Figure 4.2.8 Design configuration of Sensor 4. 43. Figure 4.2.9 The sensor, excitation and output signal. 44. Figure 4.3.1 FEMLAB model navigator. 46. viii.

(12) Figure 4.3.2 Model of Interdigital Sensor. 47. Figure 4.3.3 Window for boundary setting of rectangular block. 48. Figure 4.3.4 Window for boundary setting of sensor. 49. Figure 4.3.5 Window showing excitation and ground electrodes distinctively. 50. Figure 4.3.6 Window for create composite object. 50. Figure 4.3.7 shows the window for setting the Sub domain. 51. Figure 4.3.8 Mesh of the model. 51. Figure 4.3.9 Solve menu. 52. Figure 4.3.10 Menu to set solve parameters. 52. Figure 4.3.11 Electric field intensity for sensor 1. 53. Figure 4.3.12 Electric field intensity for sensor 2. 53. Figure 4.3.13 Electric field intensity for sensor 3. 54. Figure 4.3.14 Electric field intensity for sensor 4. 54. Figure 4.4.1 Graphical representation of sensor output voltage values for sensor 1. 56. Figure 4.4.2 Graphical representation of sensor output voltage values for sensor 1. 57. Figure 4.4.3 Graphical representation of sensor output voltage values for sensor 1. 58. Figure 4.4.4 Graphical representation of sensor output voltage values for sensor 1. 59. Figure 4.4.5 Sensor values for each material individually. 60. Figure 4.5.1 Block diagram of experimental setup. 62. Figure 4.5.2 Experimental setup. 63. ix.

(13) Figure 4.5.3 Full-wave rectifier circuit. 63. Figure 4.5.4 Voltage waveforms at different stages in the precision rectification circuit. 64. Figure 5.1.1 Image of sheep skin. 65. Figure 5.1.2 Pins of the sensor. 66. Figure 5.1.3 Sheep skin labelled into five zones. 66. Figure 5.1.4 Sensor with skin placed over it. 67. Figure 5.2.2 Sensor output voltages at each position of various samples for Group 1 Figure 5.2.3 Sensor output voltages at each position of various samples for Group 2. 70 72. Figure 5.2.4 Sensor output voltages at each position of various samples for Group 3. 74. Figure 5.2.5 (i) Looseness values for group 1 determined by two experts from LASRA. 75. Figure 5.2.5 (ii) Looseness values for group 2 determined by two experts from LASRA. 76. Figure 5.2.5 (iii) Looseness values for group 3 determined by two experts from LASRA. 76. Figure 5.2.6 Comparison of sensor output voltage with looseness values for position 4 of group 1. 77. Figure 5.2.7 Comparison of sensor output voltage with looseness values for position 4 of group 1. 77. Figure 5.2.8 Comparison of sensor output voltage with looseness values for position 5 of group 1. 78. Figure 5.2.9 Comparison of sensor output voltage with looseness values for position 5 of group 1. 78. Figure 5.2.10 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 1. 79. Figure 5.2.11 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 1. 79. Figure 5.2.12 Comparison of sensor output voltage with looseness values for average of all positions of group 1. 80. x.

(14) Figure 5.2.13 Comparison of sensor output voltage with looseness values for average of all positions of group 1. 80. Figure 5.2.14 Comparison of sensor output voltage with looseness values for position 4 of group 2. 81. Figure 5.2.15 Comparison of sensor output voltage with looseness values for average of position 4 of group 2. 81. Figure 5.2.16 Comparison of sensor output voltage with looseness values for position 5 of group 2. 82. Figure 5.2.17 Comparison of sensor output voltage with looseness values for position 5 of group 2. 82. Figure 5.2.18 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 2. 83. Figure 5.2.19 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 2. 83. Figure 5.2.20 Comparison of sensor output voltage with looseness values for average of all positions of group 2. 84. Figure 5.2.21 Comparison of sensor output voltage with looseness values for average of all positions of group 2. 84. Figure 5.2.22 Comparison of sensor output voltage with looseness values for position 4 of group 3.. 85. Figure 5.2.23 Comparison of sensor output voltage with looseness values for position 4 of group 3.. 85. Figure 5.2.24 Comparison of sensor output voltage with looseness values for position 5 of group 3.. 86. Figure 5.2.25 Comparison of sensor output voltage with looseness values for position 4 of group 3.. 86. Figure 5.2.26 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 3.. 87. Figure 5.2.27 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 3.. 87. Figure 5.2.28 Comparison of sensor output voltage with looseness values for average of all positions of group 3.. 88. xi.

(15) Figure 5.2.29 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 3.. 88. Figure 5.3.1 Leather with marked positions. 89. Figure 5.3.2 Skin area of one of the positions with 5 holes in it. 90. Figure 5.3.3 Comparison of size of the hole with 10 cents coin. 90. Figure 5.3.4 Comparison of thickness with sensor voltage before tanning. 91. Figure 5.3.4 Comparison of thickness with sensor voltage before tanning. 91. Figure 5.3.4 Comparison of thickness with sensor voltage before tanning. 92. Figure 5.3.5 Comparison of looseness with sensor voltage before tanning with skins arranged in the increasing order of thickness.. 93. Figure 5.3.5 Comparison of looseness with sensor voltage before tanning with skins arranged in the increasing order of thickness without considering few samples.. 93. Figure 5.3.6 Comparison of looseness with sensor voltage for the samples having same looseness arranged in increasing order of thickness.. 94. Figure 5.4.1 Comparison of sensor output voltage with looseness values for position 4 of group 1. 95. Figure 5.4.2 Comparison of sensor output voltage with looseness values for position 4 of group 1. 95. Figure 5.4.3 Comparison of sensor output voltage with looseness values for position 5 of group 1. 96. Figure 5.4.4 Comparison of sensor output voltage with looseness values for position 5 of group 1. 96. Figure 5.4.5 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 1. 97. Figure 5.4.6 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 1. 97. Figure 5.4.7 Comparison of sensor output voltage with looseness values for average of all positions of group 1. 98. Figure 5.4.8 Comparison of sensor output voltage with looseness values for average of all positions of group 1. 98. xii.

(16) Figure 5.4.9 Comparison of sensor output voltage with looseness values for position 4 of group 2. 99. Figure 5.4.10 Comparison of sensor output voltage with looseness values for average of position 4 of group 2. 99. Figure 5.4.11 Comparison of sensor output voltage with looseness values for position 5 of group 2. 100. Figure 5.4.12 Comparison of sensor output voltage with looseness values for position 5 of group 2. 100. Figure 5.4.13 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 2. 101. Figure 5.4.14 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 2. 101. Figure 5.4.15 Comparison of sensor output voltage with looseness values for average of all positions of group 2. 102. Figure 5.4.16 Comparison of sensor output voltage with looseness values for average of all positions of group 2. 102. Figure 5.4.17 Comparison of sensor output voltage with looseness values for position 4 of group 3.. 103. Figure 5.4.18 Comparison of sensor output voltage with looseness values for position 4 of group 3.. 103. Figure 5.4.19 Comparison of sensor output voltage with looseness values for position 5 of group 3.. 104. Figure 5.4.20 Comparison of sensor output voltage with looseness values for position 4 of group 3.. 104. Figure 5.4.21 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 3.. 105. Figure 5.4.22 Comparison of sensor output voltage with looseness values for average of positions 4 and 5 of group 3.. 105. Figure 5.4.23 Comparison of sensor output voltage with looseness values for average of all positions of group 3.. 106. Figure 5.4.24 Comparison of sensor output voltage with looseness values for average of all positions of group 3.. 106. Figure 5.4.25 Comparison of looseness with sensor voltage after tanning with. 107. xiii.

(17) skins arranged in the increasing order of thickness. Figure 5.4.26 Comparison of looseness with sensor voltage after tanning with skins arranged in the increasing order of thickness.. 107. Figure 5.4.26 Comparison of looseness with sensor voltage after tanning with skins arranged in the increasing order of thickness. 108. 108. Figure 5.5.1 Comparison of actual looseness with calculated looseness with skin samples arranged in increasing order of thickness.. 111. Figure 5.5.2 Comparison of actual looseness with calculated looseness with skin samples arranged in increasing order of thickness.. 111. Figure 5.5.3 Comparison of actual looseness with calculated looseness with skin samples arranged in increasing order of thickness.. 113. Figure 5.5.4 Comparison of actual looseness with calculated looseness with skin samples arranged in increasing order of thickness.. 113. Figure 7.1 Microcontroller. 115. xiv.

(18) LIST OF TABLES Table 4.3.1 Capacitance values of four sensors. 55. Table 4.4.1 Sensor output voltage values for sensor 1. 56. Table 4.4.2 Sensor output voltage values for sensor 1. 57. Table 4.4.3 Sensor output voltage values for sensor 1. 58. Table 4.4.4 Sensor output voltage values for sensor 1. 59. Table 5.1.1 Sensor results for various samples. 69. Table 5.2.2 Results for group 2. 71. Table 5.2.3 Results for group 3. 73. Table 5.5.1 Scaling factor and calculated looseness values for skins before 110. 110 tanning. Table 5.5.2 Scaling factor and calculated looseness values for skins after tanning 112 Table 6.1 Relationship between ADC values and Looseness values. xv. 116.

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(20) CHAPTER 1 INTRODUCTION 1.1. Introduction In this research, interdigital sensors were used to identify the looseness characteristics of sheep skins in a non-destructive or non-invasive way. Non-Destructive Testing (NDT) is important as it would not alter the chemical or physical properties of the material under test. Non-Destructive Testing is used in various fields which are explained in detail in the nest section.. 1.2 Non-Destructive Evaluation NDT is applied at almost any stage in the production or the lifecycle of components. In this report, we developed an interdigital sensor based sensing system that could measure the looseness in sheepskin in a non-destructive and non-invasive method. Materials and manufactured products are usually tested before delivery to ensure their performance, safety, durability and quality. In some scenarios, the materials and products need to be tested, not only at the production stage but also at specified intervals during their performance stage like examination of critical regions in structures and components used in aircraft that could be affected by fatigue, components used in chemical processing etc. It is essential that any test made on the product should not alter its properties or performance. Any technique which meets the above requirements can be referred to as Non-Destructive Evaluation (NDE) or also called as Non-Destructive Testing (NDT). NDE is the assessment procedure which doesn’t alter the material under test physically or chemically. NDT testing has gained importance due to great change in technology where risks are high and strict precautions are required [1]. Previously, tests were limited to audible or visible inspections. Audible inspection involved striking a casting or forging with an iron bar to produce a sound to determine if a vessel or a structure got a crack or not. Requirement of fast performance and. 1.

(21) durable materials resulted in development of new materials and redesign the structures to reduce their weight and increase strength. All the above requirements have led to widespread applications of non-destructive testing in various fields to ensure that safety limits are not exceeded [1]. Some of the reasons for conducting NDT techniques can be stated as:  To avoid defects in the materials likely to cause failure,  To ensure the dimensions of a component or structure,  To determine the structural and physical properties of a material,  To meet the health and hygiene standards,  To avoid physical or chemical alteration of material or component under test. NDT can be performed on metals as well as non-metals and the method of testing usually depends on factors such as the type of the material, its dimensions, position of interest within the material under test, interior or exterior defects. Some of the NDT methods are:  Visual and Optical Testing [2,3]: Visual examination, computer controlled camera image recognition.  Radiography testing [4]: X-rays, gamma rays and neutron beams.  Magnetic particle testing [5]: Inducing magnetic field in a ferromagnetic material.  Ultrasonic testing [6]: transmission and detection of ultrasonic sounds.  Penetrant testing [7]: Coating objects with fluorescent dyes or visible dye.  Leak testing [8]: Gauge measurement, liquid and gas penetration, soap bubble tests and electronic listening devices.  Acoustic emission testing [9]: detection of acoustic emissions  Electromagnetic testing [10]: eddy current inspection, remote field testing, flux leakage and barkhausen noise inspection.. 2.

(22) For complete inspection of an object, combination of two or more above methods is generally required [1]. The most commonly used methods are ultrasonic testing, Xradiography, eddy current testing, magnetic particle inspection and dye penetrating. The international standards organizations usually give more attention to the above inspecting methods. Some of the NDT applications in various industries include:  Power Stations [11]: NDT can be used for inspecting generator turbine for existence of any crack, fatigue etc.  Metal Industry [12]: for the inspection of cracks, defects and any other flaws and their characterization, wall thickness, quality assurance, fatigue estimation, determination of hardness and coating thickness testing etc in steel production and steam and pressure vessel construction.  Petrochemical Industry [13]: used to detect surface breaking defects through paint and other coatings of various thicknesses, and then accurately size them in terms of length and depth.  Transportation [12]: for measuring the life of the tracks in railways.  Food Industry [14]: estimating dielectric properties of various types of meat.  Medical Sciences [15]: To measure the thickness of coating on the tablets  Civil Engineering [16]: Inspection of concrete structures, bridges, cracks or decrease in strength due to aging problems.  Aviation Industry: detecting corrosion and disbonds in large areas of lap joints in aircrafts [17]. For fatigue estimation in parts of aircraft [1].  Pipe Inspection: For inspection of pipes carrying gases [18], milk, water, oils etc. It is evident that NDT techniques are being applied successfully in various fields. Even, picking up the fruit at regular supermarket could also be regarded as non-destructive testing which involves visual inspection. Most of the sensors can be employed in nondestructive evaluation of the materials. In the case of interdigital sensors (discussed in detail in chapter.3) the material to be tested is placed over it or arranged between its parallel plates to investigate its characteristics. In this report, non-destructive testing was extended to. 3.

(23) inspection of properties of sheep skin during different stages (discussed in chapter.2) of converting raw pelts into finished leather using novel interdigital sensor which is explained in detail in chapter. 3.. 1.3 Sensors A Sensor is a device that measures a physical quantity and coverts it into a signal that could be read by an instrument or an observer. A sensor is capable of detecting a change in physical conditions like temperature or thermal conductivity or change in chemical concentration and they should be able to convert the detected change into a measurable unit. Sensors are an important part of any measurement or an automation application. A good sensor should be sensitive to the measured property and should not influence the measurements of material under test or its properties. Sensors are used in everyday objects such as infrared automated door openers, touch-sensitive elevator buttons and lamps which dim or brighten by touching the base. There are also innumerable applications for sensors of which most people are never aware. When selecting a sensor following things should be considered:  Accuracy: It should provide accurate readings  Cost: The cost of sensor should be economical  Range: The minimum and maximum range of the output value of sensor  Resistance to factors affecting the sensor: The factors that influence the reading of the sensors  Repeatability: Able to repeat the same experimental values  Precision: Should be able to detect the smallest change in the measurement.  Life expectancy: Sensor should have a good durability.  Quick response: real time monitoring  Low operation and maintenance costs  Meets safety standards  Continuous operation  Ease of calibration  Easy Interfacing: Should be adaptable to various interfacing devices. 4.

(24) Sensors are an integral part of everyday life. Sensors may be broadly classified as thermal, electromagnetic, mechanical, chemical, optical, ionizing or acoustic types, depending upon their fabrication and the physical quantity that they measure. For example, chemical sensors respond to the change in the concentration of a chemical or recognition of a chemical [19]. Biosensors respond to the micro-organisms that either stick to it or grow on the surface of the sensors [20]. In the process of detecting or responding to certain factors, the sensors produce either a current or voltage signal. These signals often need to be conditioned before processing. The processing can be efficiently done using a digital data acquisition system. Sensors are classified into passive and active sensors [21]. Passive sensors can only be used to detect parameters when the naturally occurring energy is available. For all reflected energy, this can only take place during the time when the sun is illuminating the Earth. Energy that is naturally emitted for example, thermal infrared can be detected day or night, as long as the amount of energy is large enough to be recorded. Active sensors provide their own energy source for illumination. The sensor emits radiation which is directed towards the target to be investigated. The reflected radiation from the target is detected and measured by the sensor. Advantage of active sensor is that, it could obtain the measurements anytime regardless of day or season. Active sensors are capable of examining wavelengths that are not sufficiently provided by the sun, such as microwaves. However, active systems need a large amount of energy to adequately illuminate the target. Sensors are calibrated for certain conditions and are capable of reporting changes at certain speeds. Sensors will be more useful when they are amplified [22]. Sensors could be either read directly (e.g. mercury thermometer) or should be interfaced with an indicator (e.g. an analog to digital converter, a computer to display the value, or a display setup or even a microcontroller for a digital display of values) that would make it easier to read the values. In general, sensors are mostly either analog or digital sensors. Analog sensors produce an output signal that is continuous in both time and magnitude. Physical variables that are continuous in nature such as air flow, temperature and speed can be measured by analog sensors. Disadvantages of analog sensors are electric system noise, cross talk and as well as. 5.

(25) performance reductions over transmitting the analog signal over large distances. Examples of analog sensors include potentiometers, resistance temperature devices (RTD), microphones and strain gauges. Digital sensors generate what is called a 'Discrete Signal'. This means that there is a range of values that the sensor can output, but the value must increase in steps. There is a known relationship between any value and the values preceding and following it. 'Discrete Signals' typically have a stair step appearance when they are graphed on chart. The output of the digital sensor must be compatible with the digital receiver. Examples of digital sensors include switches, infrared detectors and position encoders. Sensors usually output one of two types of signal, an analog signal or a discrete signal. Microcontrollers usually deal with discrete or digital signals. An analog to digital converter allows the output of an analog device to be used by a Microcontroller. Many Microcontrollers have A/D converters built in. Interfacing will depend on what type of output a sensor provides and care should be taken to not to create a path that allows too much current to flow. Current limiting resistors are important in interfacing Microcontrollers to sensors. Sensors are critical to today’s society since they provide the connection between the real world and the world of process controllers and computers. The over all accuracy and reliability of the control system would depend on the sensors accuracy. Sensors have played a major role in improving energy efficiency, service, product quality and reducing emissions [23]. Sensors are integral when it comes to controllability, reliability and profitability of a process [22, 23]. Sensors technology follows a pattern of continuous development and many prototypes will be introduced depending on the requirement in various fields. While manufacturing a sensor, the factors that manufacturers would consider are cost reduction, reliability,. system. compatibility,. safety. in. noninvasive/nonintrusive design.. 6. hazardous/hostile. environments. and.

(26) Many of the sensor technologies that are in use today apply complex mixtures of several different materials, where the principles of functionality of each component is not known or well understood [24]. Furthermore, aging of the sensors could also result in inaccuracy. So, for the successful implementation of novel sensor technologies, it is important to have a good understanding of sensing mechanisms and their degradation behaviour which would aid in the development of advanced, affordable, reliable, and novel technologies that would have a major impact on the society. A firm understanding of the material characteristics is also important in selecting the appropriate combination of sensing elements to achieve selectivity in complex array structures [24]. There is a high demand for novel sensors that are able to withstand and perform in extreme hostile/hazardous environments [25]. Novel technologies which are reliable in extreme environments are continuously being researched and developed, however the sensing requirements are becoming much more demanding. It is important to understand the sensing mechanisms and how they operate in each case. The miniaturization and faster processing of signals of such devices and systems seems to be the next step in sensor technology and with the aid of nanotechnology, MEMS it does not seem a distant dream. Dielectric Analysis (DEA) techniques could be used for testing materials that have poor electrical conductivity [26-31]. DEA is based on the principles of electrostatics. DEA techniques are non-destructive and can be used to relate molecular motions observed in an electric field, to a variety of polymeric properties. In electroquasistatic applications, capacitive sensing dielectrometry is used to provide information of materials with poor conductivity [28]. Depending on the measurements of materials electrical properties such as dielectric constant, conductivity, loss tangent or complex permittivity, characteristics of the materials such as layer thickness, thermal conductivity, presence of defects , porosity can cure state can be determined.. 7.

(27) 1.4. Objective of the research Farmers, companies and researchers all around the world are looking for better methods to improve the quality of leather. There is always a demand for good quality leather and the companies there by farmers are paid according to the quality of leather they supply. According to Wikipedia, tanning is the process of of converting putrescible skin into nonputrescible leather and once the tanning process is finished you cannot reverse it. Tanning process is explained in detail in chapter, 2. Value of the leather is determined by its quality and looseness is one of the quality attributes that determines its value [32]. A sheep skin is made up of collagen fibre which enables the skin to be flexible. The collagen fibres needs sufficient space around it so that they can move in relation to other collagen fibre for free movement, however an extra space which is more than required between them results in looseness. In figure 1.4.1, cross section of leather with more visible spaces represented in white color is shown. In figure 1.4.2, cross section of leather with little or less space is shown. Here, more looseness in leather means inferior quality and a good quality leather should be stronger or mire tight. In comparison to both the figures 1.4.1 and 1.4.2, 1.4.1 is inferior quality and 1.4.2 is good quality leather.. Figure 1.4.1: Cross section of loose leather with extra spaces between the fibres (LASRA). 8.

(28) Figure 1.4.2: Cross Section of tight leather with less space between the fibres (LASRA) Looseness can be treated by altering the leather making process which would depend on the amount of looseness present in the skins. New Zealand lamb skins have a reputation for looseness, a condition arising from the open fibre weave of the skin which can lead to the formation of coarse and unattractive creases in the leather [32]. Aggressive processing of the skin in the early stages of leather making can exacerbate the condition by removing too much of the leather making substance. Under processing of sheep skins may not result in desired quality leather. In either ways it is harmful for the reputation of the parties involved in the production and also reduces the value of the leather. So, for the production of better quality leather you need to know the proper leather making process required for that particular sheep skin lot. It is difficult to identify the looseness in skins at an early processing stage in order to take corrective action, as the extent of the problem is really evident after tanning the skins. So, some means of identifying looseness in skins at the pickling stage would allow the processes to be modified to reduce the damage. The aim of the research was to develop a noninvasive and non-destructive interdigital sensor based sensing system to measure the looseness in skins during early stages that could be used as a production control tool for monitoring product quality which is economical and easy to use at the same time.. 9.

(29) 1.5. Research on Skin Property Estimation Some ideas were developed to measure the looseness in hides. The Shoe and Allied Trades Research Association (SATRA) in the UK developed a “break” scale [33]. This involves bending the hide around a pre-shaped mandrel and observing the creasing on the inward-folded grain surface which was compared with a scale and graded. However, this scale cannot be applied to ovine skins due to the differing character of ovine leather. There had been some unsuccessful attempts to determine the looseness in sheep skins earlier. There were attempts to develop a non-destructive device for measuring looseness at an intermediate stage of production of leather from lamb pelts. Successful attempts at nondestructive testing of cattle hides was extended to sheep skins. The tests on cattle hides were carried out on one side only by applying concentric twisting forces, but when applied to relatively thin sheep skins the forces caused the whole thickness of the skin to move, creating ridges and folds which does not have any relationship to the physical characteristics of the grain surface [34]. There was a limited success with destructive techniques for measurement of looseness in ovine skins [35]. Leather and Shoe Research Association (LASRA) conducted a study to compare the objective and subjective methods for assessing the looseness in sheep skins [36].. •. Subjective assessment: An experienced leather technologist graded sixty pelts of varying skin character and sorted them into four different grades of equivalent looseness. Each grade was assigned a number from one (least looseness) to four (extreme looseness) as a subjective assessment.. •. Objective assessment: Objective assessment was based on the break scale developed by SATRA which was redeveloped to suit the ovine skins. Under this method, an area was chosen which was 100 mm apart from midline of the skin and 100 mm from the belly edge off which eight sites were picked for looseness assessment. At each assessment site, the skin was folded perpendicular to the backbone to produce crease. The appearance of the crease was then compared with the LASRA. 10.

(30) looseness scale and was graded from 1 (no looseness) to 8 (extreme looseness). This procedure was repeated for all eight sampling sites and mean value of grades at all sample sites was considered as the looseness grade of that particular skin. There was 95% confidence interval of objective looseness for each subjective looseness grade. Objective ranking and Subjective ranking had a correlation between the rankings of 0.830. Acoustic emission was tested to check the disruption of adhesions in calfskin and mature cattle hides. Tensile strength of the bovine leather was examined and determined that acoustic emission can be used to detect failure processes in leather before it actually tears or is substantially weakened [37]. Later, acoustic emission sensors were used to measure the softness in leather. A rotational acoustic sensor was rolled across the leather samples to collect their AE quantities such as waveforms, frequency hits, counts and energy. Sound waves produced by the fibrils, fibres and fibre bundles due to deformation of leather (squeezed, pressed, torn or stretched) caused by an external force are detected by an acoustic sensor and converted into electric signals. The higher AE energy was an indication of stiffer leather and stiffer leather is prone to bad grain break. It was reported that stiffer leather samples produce more counts than softer samples and softness, grain break and tensile strength of the leather could be measured non-destructively using the setup [38].. 1.6. Organization of the Thesis This thesis is organized into six chapters. In chapter 1, theory of Non-destructive evaluation, an introduction to sensors and objective of the work is presented. Chapter 2 describes the evaluation of quality of leather, looseness and leather making process. In chapter 3, introduction to Interdigital sensors, their working procedure, modelling the sensors using FEMLAB and comparison of sensors is shown. Chapter 4 describes the experimental set-up and the interfacing of sensor signal to a data acquisition system. Chapter 5 includes experimental observation and results. Finally, the work has been concluded in chapter 7.. 11.

(31) CHAPTER 2 LEATHER: EVALUATION OF QUALITY 2.1. Introduction Looseness is a major problem in the leather industry. It is apparent as coarse wrinkles in the finished leather and traces can be found in early processing or pickled stages. There are a variety of possible causes of looseness in skins such as putrefaction of the raw material, overliming, excessive swelling during liming, excessive mechanical action, inadequate penetration of fat liquor but the effects are not apparent until the leather is tanned. The effects of looseness can be ameliorated by modifications to the tanning process but these are expensive. New Zealand pelts have a reputation for being loose and the price of the pelt is discounted with increase in looseness. If the processor can keep track of looseness in the pelts at the pickled stage or during the early stages, he can alter the tanning process to correct the problem and produce leather of desired quality depending on its end use. So, we designed a non-destructive and non-invasive method of measuring the looseness values in sheep skins using interdigital sensor based sensing system. In this chapter, sheep skin structure, looseness and factors affecting looseness, tanning process, and different types of leathers are discussed in detail. All the information regarding skins and skin samples were supplied by LASRA (Leather and Shoe Research Association New Zealand).. 2.2. Structure of Sheep skin When an animal is alive its skin has the natural properties of flexibility, toughness and being waterproof, but, when animal dies the skin looses these properties. After death of the animal, if the skin is wet it is susceptible to bacterial attack and when dry the skin become inflexible making them useless. Animal skin that has been processed to retain its flexibility, toughness and waterproof nature is known as leather. Tanning is the process which involves converting raw skin to leather. Tanning converts putrescible biological material into a stable material which is resistant to microbial attack and has enhanced resistance to wetness and heat [39]. Tanning permanently alters the structure of skin so that it can not ever return to rawhide.. 12.

(32) The cross section of sheep skin is shown in figure 2.2.1. In order to understand the technology involved in skin processing, knowledge of sheepskin structure and composition is important. The skin is complex topic because of the functions it has to perform for the animal. The major component of the skin is the fibrous protein collagen which accounts for around 77% of the fat-free dry weight of the skin and is the source of the tensile strength of the skin. Unlike keratin, the protein of hair and wool, collagen has few natural permanent cross-links which can give it thermal stability. As a consequence the skin structure is irreversibly disrupted by increasing the temperature to around 66οC. One of the main purposes of tanning is the stabilisation of the collagen structure by the formation of strong, permanent cross-links between the protein chains and during tanning process processed leather will able to withstand temperatures to about 100οC. The type and thermal stability of the cross-link depends on the process involved in tanning.. Figure 2.2.1: Cross section of sheep skin (LASRA). A sheep skin is composed of three major identifiable parts – the epidermis, the grain layer and the corium. The disruption of the collagen network causes the grain layer to be weak and despite accounting for half the thickness of the skin it cannot contribute much to skin strength. Collagen fibres are thin and tightly woven together in the grain layer whereas. 13.

(33) in the corium the collagen fibres are coarser, stronger and tightly woven together. The grain and corium layers structures are affected differently while processing because of the differentiating rates of penetration of different reagents because of their varying respective structures. Between the grain layer and the corium and there is frequently a layer of cell that contains fat which becomes distinctive with age. The presence of this fat can discolour the finished leather as well as produce unpleasant odours, so it is important to get rid of the fat while processing the sheep skin [40]. As discussed in chapter 1, looseness can be termed as extra space between the collagen fibres. Looseness can also be referred to a condition of incompact fibre weave and affects the skin thickness as a whole rather than just corium or grain or the grain corium junction. Looseness can also be detected in skins in the pickled stages or early processing stages. Most skin fat could be found at the grain/corium junction and the zone of weakness or looseness of the leather could not be related to the presence of fat in those skins [41]. There are other factors such as breed, age, bacterial damage etc which result in looseness. The above factors are discussed later in this chapter. In figure 2.2.2, images four different leather samples are shown. Each sample showed in figures 2.2.2 (a), (b), (c) and (d), have different concentrations of fat present in them. All the images show the fat coloured in red as they have been stained with Sudan III. The looseness can be observed as white space in these images. Image 2.2.2(a) has less fat which would affect the colour and odour of the finished leather and less looseness which makes it good quality leather. Image 2.2.2(d) got lots of fat as well as more looseness which makes it inferior quality leather.. 14.

(34) 2.2.2(a). 2.2.2(b). 2.2.2(c). 2.2.2(d). Figure 2.2.2: Leather samples with fat cells and looseness shown (LASRA). 15.

(35) 2.3. Looseness Leather is usually graded depending on its end use. Looseness results in the appearance of unpleasant creasing, especially when the leather is folded or flexed. The greater the looseness the coarser the creases are on the leather. Looseness deteriorates the quality as well as the value of the leather. Looseness is not restricted or confined to just one area or specific site of the leather, but is a function of leather as a whole [36]. Looseness in leather is measured by holding it and manually pulling it in opposite direction. Depending on the creases appearing on the skin it is compared with a scale (LASRA) as shown in figure 5 and graded from 1 to 6, where 1 being least loose and 6 being more loose [42]. Leather with looseness values in between 1 to 3 are regarded as good quality leather and quality as well as value degrades from 4 towards 6.. Figure 2.2.3: Looseness scale determined by LASRA New Zealand pickles lamb pelts are characterised by two key features that reflect strongly on their ease of processing by overseas industries and their value in the international garment leather trade. These features are the high level of natural fat in the pickled pelt or skin at early processing stage and the property known as looseness. The key focus of lamb pelt processing is the production of flat pelts for garment use and the avoidance of looseness. Looseness in leather is a major problem in the industry. Looseness eventuates from the natural structural nature of the skins derives from wool breeds and is influenced by the fat and protein removal that occurs in processing. Controls can be exercised over looseness but there is no means of measuring this key feature in production to be able to confirm its extent of presence or absence. Looseness is commonly determined after it had been processed into leather. A production that attracts a reputation for looseness suffers in the market as a consequence.. 16.

(36) 2.4 Factors affecting Looseness There were concerns over the degree of looseness in pelts produced in New Zealand which is often blamed on bating however there are a variety of factors that affect looseness. In sheepskins looseness can also arise if the skins are particularly fatty at the junction of the grain and corium. Removal of the fat during degreasing leaves a void between the two layers, which is exacerbated by subsequent mechanical action. Looseness can lead to floating grain and the presence of floating grain. Floating grain: It is a result of a failure in adhesion between the grain and the corium. This occurs due to the sharp change in structure between the grain and the corium layers accentuated by localised deposition of fat at the grain/corium junction, which lead to a zone of weakness. Looseness: It is a condition of in-compact fibre weave and substance loss. Looseness pertains to the whole structure of the skin whereas floating grain is a function of a specific site on the skin. There are many different causes for looseness, there is no simple formula or single process to provide a solution. If the problem suddenly occurs in the production, then any change in processing such as change in chemicals or treating times etc have to be rechecked. Raw skins need to be checked for any traces of bacteria. Equipments also need to be checked for example, incorrect operation of the timer on the processing drum, or breaking down of water thermostat. It is advised not to overcome the problem by adding more chemicals, but to address the cause, ‘leather should not be loose, there must be something causing it’ [32]. An ability to objectively measure looseness at the pickled stage would further increase the producers ability to maintain the market appeal for the product and if required manipulate processing steps in response to the changing nature of the raw material and changing nature of the end requirement.. 17.

(37) A basic guide for producing good quality leather with low looseness values would be [32]:  Use good quality raw hides and skins  Minimise swelling during liming  Avoiding prolonged running times of drums as well as excessively fast speed drums  Proper conditioning prior to staking or dry drumming  Adequate fatliquoring  Ensure the desired action of chemicals With consideration to all the above factors, there are other factors that influence looseness, they are:.  Rawstock Effects  Processing Effects  Storage Effects. •. Rawstock Effects: Several factors such as breed and animal health can be placed under Rawstock effects..  Breed The breed of the sheep could be a factor that affects the looseness of the pelts. Research work shows that amongst three breeds Cheviot, Drysdale and Romney, Cheviot produced tighter leather compared to Drysdale and Romney with respect to looseness [43]. The predominant sheep breed in New Zealand is the Romney-cross which is quiet prone to loose skin..  Animal Condition Usually an animal in poor condition is anticipated to have a looser pelt, however, it was found that level of feed did not have an effect on the looseness of the lamb pelts [43].. 18.

(38)  Age The age of the stock at slaughter also has a significant effect on the looseness of the leather. It was found that increasing age at slaughter decreased looseness of lamb pelts [44]..  Bacterial Damage Looseness in pelts is also affected by the bacterial effects on the skin. The effect appears similar to over bating and is caused by bacterial and skin enzymes breaking down the skin structure. This damage will occur if the skins are not adequately preserved from the time they are removed from the carcase to the commencement of processing the fellmongery. Skins must be promptly chilled after slaying and kept cold both until and during transport to the processing stations.. •. Processing Effects: A variety of factors that involve processing can affect looseness. Looseness caused by the processing is the major contributor to the problem..  Painting Two factors may affect looseness in painting process  Paint concentration  Time held in painted condition Paint concentration affects the degree of swelling in the collagen matrix. The higher the concentration of sulphide, greater the alkalinity in liming stage which results in extra swelling. This swelling can lead to disruption of the grain/corium junction and floating grain. The time held in the painted condition is also an important factor as the pelt substance is being dissolved and opens up with time. The longer a pelt is held in the painted condition the looser is the pelt.. 19.

(39) Swelling aids in the splitting up of collagen fibres, allowing the removal of cementing substances within the skin. Removal of these cementing substances enhances the properties of finished leather like softness, flexibility, strength and also aids dyeing. Excessive swellings, however, increases the effects of drum/processor damage, as well as leading to increased mottle, double hiding and looseness. The rate of swelling is also a factor, if swelling occurs gradually over a long period of time the skin structure can potentially adjust to the change in swelling gradually. However, if swelling occurs rapidly distortions between grain and corium may become apparent leading to floating grain. Addition of sodium sulphate or sodium carbonate significantly reduces swelling during depilation and lower application rates of more concentrated depilation will significantly reduce swelling. Swelling factors do significantly impact the properties of pelts in terms of flatness and looseness..  Liming Sulphide concentration and liming time are important factors that would affect looseness. Float length exerts a strong influence on degree of swelling. At a certain pH level, the degree of swelling increases along with the water quantity. Too much mechanical action of the processing drum during liming will also promote looseness. Generally liming drums are only run for short periods (5minutes every hour) and at low speeds (12 rpm)..  Bating Overbating affects the looseness of pelts and it is not always easy to detect overbating. Bating process is also dependent on the previous processes carried out. Required bating would depend on how the skin was painted and limed because of the amount of inter-fibrilar matter remaining and degree of flatness to be produced in the end product. Increasing the degree of bating increases the strength of leather to a maximum value and further bating would decrease the strength.. 20.

(40) •. Storage Effects: Work by Leather and Shoe Research Association (LASRA) has shown that storage of pickled pelts affects the soluble protein substance of the pelts. Acid hydrolysis from the pickle acid in a pelt causes break down of the collagen and there by reduces the quality of the product. Looseness in pelts increases with storage time [45].. 2.5. Processing of Sheep skin. When an animal is alive, its skin is soft, flexible, very tough and hard wearing: it has the ability to allow water vapour to pass out, but it will not allow water in. When the skin dies it loses these characteristics: if it is kept wet it rots, and if it is dried it goes hard and brittle. The process of tanning is to retain the skin’s natural properties as stated above, to chemically process it and at the same time stabilise its structure so that it will no longer be subject to putrification. . Thus leather is animal skin that has been processed to retain its natural properties. Skin is made up of many bundles of interwoven protein fibres called collagens that are able to move in relation to one another when the skin is alive. When the skin dies, these fibres tend to shrivel and stick together. Essentially, the purpose of tanning process is to permanently fix the fibres apart by chemical treatment, and to lubricate them so they can move in relation to one another. Well tanned leather, therefore, retains the properties of flexibility, toughness and wear. It also continues to ‘breathe’, allowing water vapour to pass through but remaining reasonably water-proof. It is this characteristic which accounts for the comfort of genuine leather shoes and clothing [46]. The process of converting a raw skin into imputrescible, removing the unwanted matter from the structure and stabilise and preserve it, whilst retaining the useful properties is the ideology behind tanning. Modern tanning process is usually done in 8 steps that involve unhairing, liming, deliming and bateing, pickling, tanning, neutralizing, dyeing and fatliquoring, drying and finishing. The whole process is a long time consuming process which could take from 1 day up to 6 or 7 days.. 21.

(41) 2.6. TANNING IN ANCIENT HISTORY Tanning has come a long way. Many years ago there was a saying “Every animal has just enough brains to preserve its own hide”. It was due to the methodology followed by the ancients, which involved animal brains in the tanning. In ancient history leather was used for water bags, harnesses, boats, armour, quivers, scabbards, boots and sandals. Skins were first soaked in water to clean and soften them and then they would pound and scour the skin to remove any remaining flesh and fat. Then, the hair was removed by either soaking the skin in urine, painting it with an alkaline lime mixture, or simply letting the skin putrefy for several months then dipping it in a salt solution The hair was scrapped off with knife after the hair fibres were loosened. Once the hair was removed the skin was bated by pounding dung into the skin or soaking the skin in a solution of animal brains. Sometimes the dung was mixed with water in large vat and the prepared skins were kneaded in the dung water until they became supple, but not too soft and the kneading could last two or three hours. Cedar oil, Alum or tannin was applied to the skin as a tanning agent in variation to the regular process [47]. There were no means to measure the looseness or control the looseness in the leather produced.. 2.7. MODERN METHODS OF TANNING Using modern technology animal skins are converted to leather in an eight step process as follows [46, 48]: Step 1 – Unhairing: The animal skins are steeped in an alkali solution that breaks down the structure of the hair at its weakest point (the root) and so removes the hair. The hides are placed in vats containing aqueous solution of unhairing chemicals and agitated to loosen and remove the hair [49, 50]. The rate of unhairing depends on the temperature of the mixture of chemicals, chemical concentration and amount of agitation. These parameters would depend on the end use of hair such as being used as an. 22.

(42) end product or treated as waste. The unhairing process does not remove or loosen all of the hair on the hide. Step 2 – Liming: The hairless skin is immersed in a solution of alkali and sulphide to complete the removal of the hair and to alter the properties of the skin protein (collagen) [50]. The collagen becomes chemically modified and swells, leaving a more open structure. During unhairing, hide absorbs enough moisture to increase its thickness about two-fold, referred as alkaline swelling and this may not occur uniformly throughout the hide. During liming process, further swelling of fibrous structure takes place and enables the separation of the fibres and fibrils from one another and opens up the whole structure [51]. In liming, skins are immersed in the solution for whatever time is necessary to produce the desired effects. The process of unhairing finishes during the liming process and collagen is modified considerably. Too much mechanical action of the processing drum during liming can result in looseness [45]. Step 3 – Deliming and Bating: The skin structure is then opened further by treatment with enzymes, and further unwanted material is removed. Deliming removes residual alkaline chemicals used in the previous process and swelling is reduced. Bating separates the collagen protein fibres within the hide and destroys remaining hair roots and unwanted pigments [51]. Cleaner appearance and softer texture of pelts is achieved during this process. Increasing the degree of bating increases the strength of leather to a maximum value and further bating would decrease the strength [45]. The bating process may alone take from 1 to 4 hours however, the reaction time for the dual process, bating and liming may range from a few hours to overnight. After bating, the hides are washed to remove all unwanted substances that has been loosened or dissolved. In the past deliming and bating were separate processes, but currently these are performed simultaneously using bates consisting of deliming chemicals.. 23.

(43) Step 4 – Pickling: Pickling preserves the leather and allow the hides to be stored for longer periods of time [52]. Pickling is the most acidification process as skins are agitated in a solution of salt and sulphuric acid [53]. Skins are subjected to controlled swelling using acid and complete penetration of the acid requires few hours. Excessive swelling can result in the looseness. Step 5 – Tanning: This is the most chemically complex step. Tanning is the process whereby the hides are made into product that resists decay or putrification [54, 55]. During tanning, the skin structure is stabilised in its open form, by replacing some of the collagen with complex ions of chromium [49, 56]. Depending on the compounds used the colour and texture of the leather changes. When leather has been tanned it is able to 'breathe' and to withstand 100oC boiling water, more flexible than an untreated dead skin and builds up resistance to chemicals and abrasion [47]. Step 6 – Neutralising, Dyeing and Fat Liquoring: After tanning leather is neutralised to remove unwanted acids to prevent deterioration during the drying process, and prepared for next processes: dyeing and fat liquoring [56]. Typical dyes used for dyeing are aniline-based. Variations in hide pigmentation and depth of colour penetration are factors that affect colouring of the hides. Once the leather is dyed, fat liquoring is the process in which 'tanned' fibres are treated with reactive oils, which attach themselves to the fibrous structure, and lubricate them so that they can move readily in relation to one another, producing a soft, supple leather. The quantity and quality of oils used determine the firmness, flexibility and strength of the final product. Step 7 – Drying: The leather is subjected to a drying process to remove excess moisture. As water is removed from the leather, its chemical condition is stabilised and the final properties of leather are determined [48].. 24.

(44) Step 8 – Finishing: Finishing involves applying a surface coating which would enhance the natural qualities of the skin and covers defects such as scars, horn damage, seed scars etc [51]. In case of suede leather it is buffed to give it an even texture. The main requirements of finishing process are evenness and the reproducibility of colour and adequate wear and feel properties. Animal skins that are processed in New Zealand go on to be made into a variety of leather goods, or are exported in an unfinished condition to be further treated overseas [47]. There are various tanning processes [57], some of them are,. •. Vegetable-tanned leather - is tanned using tannin and other ingredients found in vegetable matter, tree bark, and other such sources.. •. Chrome-tanned leather – is tanned using chromium sulfate and other salts of chromium.. •. Aldehyde-tanned leather - is tanned using glutaraldehyde or oxazolidine compounds.. •. Synthetic-tanned leather - is tanned using aromatic polymers such as the Novolac or Neradol types.. •. Alum-tanned leather - is tanned using aluminium salts mixed with a variety of binders and protein sources, such as flour, egg yolk, etc.. •. Rawhide is made by scraping the skin thin, soaking it in lime, and then stretching it while it dries.. 25.

(45) 2.9. TYPES OF LEATHER Leather in general is sold in three forms, Full grain, Corrected grain and the Split leather [57]. •. Full-Grain leather or Top-Grain refers to the upper section of a hide that contains the epidermis or skin layer. The hides that have not been sanded, buffed or snuffed(otherwise known as Corrected) in order to remove imperfections on the surface of the hide. Only the hair has been removed from the epidermis. The grain remains in its natural state which will allow the best fiber strength, resulting in greater durability. The natural grain also has natural breathability, resulting in greater comfort for clothing. The natural full-grain surface will wear better than other leather. Rather than wearing out, it will develop a natural "Patina" and grow more beautiful over time. The finest leather furniture and footwear are made from Full-Grain leather. For these reasons only the best raw hide are used in order to create full-grain or top-grain leather. Full grain leathers can mainly be bought as two finish types: aniline and semi-aniline.. •. Corrected-Grain leather is any top-grain leather that has had its surfaces sanded, buffed or snuffed in order to remove any imperfection on the surface due to insect bites, healed scars or brands. Top-grain leather is often wrongly referred to as corrected-grain. Although corrected-grain leather is made from top-grain as soon as the surface is corrected in any way the leather is no longer referred to as top-grain leather. The hides used to create corrected leather are hides of inferior quality that do not meet the high standards for use in creating aniline or semi-aniline leather. The imperfections are corrected and an artificial grain applied. Most correct leather is used to make Pigmented leather as the solid pigment helps hide the corrections or imperfections. Corrected grain leathers can mainly be bought as two finish types: semi-aniline and pigmented.. 26.

(46) •. Split leather is leather that is created from the fibrous part of the hide left once the top-grain of the raw hide has been separated from the hide. During the splitting operation the grain and drop split are separated. The drop split can be further split (thickness allowing) into a middle split and a flesh split. In very thick hides the middle split can be separated into multiple layers until the thickness prevents further splitting. Split leather then has an artificial layer applied to the surface of the split and is embossed with a leather grain. Splits can are also used to create suede. The strongest suedes are usually made from grain splits (that have the grain completely removed) or from the flesh split that has been shaved to the correct thickness. Suede is "fuzzy" on both sides. Suede is less durable than top-grain. Suede is cheaper because many pieces of suede can be split from a single thickness of hide, whereas only one piece of top-grain can be made.. 27.

(47) CHAPTER 3 INTERDIGITAL SENSORS 3.1. Introduction The operating principle of interdigital sensors is explained in detail along with different sensing possibilities and their applications in various fields. We have utilized four types of Interdigital sensors. These sensors are of the planar type and have a very simple structure. The interaction of these sensors on dielectric material will be discussed. The operating principle behind this sensor is based on the interaction of electric field generated by the sensor with respect to the material under test (MUT). The sensor has been fabricated using simple printed circuit board (PCB) fabrication technology. The sensing properties of the four sensors are compared for different materials to select the best sensor to conduct experiments with the sheep skins. Planar interdigital sensors were chosen for the estimation of looseness in sheep skin as the skin could be placed over the sensor and the experimental procedure would not alter the properties of the skin. So the property estimation using the interdigital sensors is considered as Non-Destructive and Non-Invasive testing.. 3.2. Operating principle of Interdigital sensors The operating principle of Interdigital sensor is same as in a parallel plate capacitor [26-28, 30, 31]. The relationship between the sensor and the capacitor can be seen in figure 3.2.1, how the transition takes place from a capacitor to the sensor [58]. There is an electric field generated between the positive and negative electrodes (instantaneous polarity) which are shown in figure 3.2.1 (a) and (b) respectively. When a material is placed on the sensor, the electric field passes through the material under test which can be observed in figure 3.2.1 (c). The dielectric properties of the material as well as the geometry of the material under test affect the capacitance and conductance between the two electrodes. The variance in the. 28.

(48) electric field can be used to determine the properties of the material depending upon the application.. Figure 3.2.1: Operating principle of an Interdigital sensor [28] Historically, the first and still the most common reason for making an interdigital electrode structure was to increase the effective length, and, therefore, the capacitance between the electrodes [28]. Possibly the earliest design of interdigital electrode structure could be found in the patent of N. Tesla, issued in 1891 [59]. In this example, each “finger” was a rectangular plate, immersed in an insulating liquid. The total capacitance of the “electrical condenser” proposed by Tesla increases approximately linearly with the number of plates. One set of electrodes are connected to an AC voltage source and act as an excitation/driving electrodes. The remaining electrodes are connected to ground. When there is a material between the electrodes, the electric fields from the driving electrodes penetrate through most of the material under testing, and then terminate on the sensing electrodes. The proximity depth of electric field lines of the sensor depend on the distance between two electrodes of opposite polarity. The electric field lines are affected by the dielectric properties of the material under test [26, 28 and 31,]. Potential difference between positive and negative electrodes are maintained constant, however, the capacitive current drawn from the source is a function of dielectric properties of the materials under test. The main advantage of using the interdigital sensor is that the electric field is only produced on the testing surface; this controls interference of field lines from outside of the testing zone, concentrating it to the material under testing [28].. 29.

(49) In figure 3.2.2, the structure of typical interdigital sensor is shown. Here, one end of electrodes are connected to AC voltage source (‘+’ terminal) also called as excitation source and other end of electrodes are connected to ground (‘-‘ terminal). In our experiments, AC voltage source was provided by a frequency generator and the current through the sensor was captured across the resistor which is connected in series with the sensor. Due to the arrangement of electrodes in this structure, it is also sometimes called comb structure and referred to as finger like pattern [28].. Figure 3.2.2: Interdigital sensor structure The extent of an electric field can be varied by changing the distance between opposing electrodes. The flow of electric field lines between electrodes for varying lengths, in between electrodes are shown in figure 3.2.3. The electric field lines corresponding to minimum separation distance between the positive and negative electrodes is ‘l1’ where the alternating electrodes are of opposite polarities (+,-,+,-,+,-) and to that for the maximum separation distance is ‘l3’ where the electrode structure is still the same but with a greater distance between them. The blue, red and green correspond to the low, medium and high pitch length respectively. So, depending upon the requirement the desired extent of electric field can be achieved by varying the length between the electrodes and the strength of the signal can be controlled by controlling the electrode pattern.. 30.

(50) Figure 3.2.3: Electric field formed between two electrodes for different pitch The length between the two adjacent electrodes of same type is referred to as spatial wavelength (λ) and ideally the penetration depth is one fourth of the spatial wavelength [28]. For the spatial wavelength of 1mm as shown in figure 3.2.4, penetration depth is as little. The spatial wavelength increases along with an increase in penetration depth linearly. D is driving electrode or the AC voltage source electrode and S is sensing electrode or the ground electrode. So varying penetration depths can be achieved by adjusting the spatial wavelength between the electrodes.. Figure 3.2.4: Penetration depths for varying spatial lengths between the electrodes. 31.

(51) Interdigital sensors can be employed in various applications depending upon the requirements. They could be used to measure the density of the material as shown in figure 3.2.5 (a), the distance between the material under test and sensor could be measured with the help of varying excitation fields as in figure 3.2.5 (b). It is also possible to identify the nonuniform or unevenly shaped materials using the interdigital sensors as shown in figure 3.2.5 (c) and they are also very good moisture sensors, as shown in figure 3.2.5 (d). Thus an interdigital sensor can not only measure the dielectric properties of a material but also the density, shape of the material.. Sensing Density Figure 3.2.5(a): Sensing the material density [60]. Sensing Distance Figure 3.2.5(b): Measure the distance between sensor and the material [60]. 32.

(52) Sensing Texture Figure 3.2.5(c): Track the structure of the material under test [60]. Sensing Moisture Figure 3.2.5(d): Sensing the moisture [60] Extensive use of interdigital electrodes for sensing applications started in the 1960s [61] along with other forms of coplanar electrode structures [62]. Later, independent dielectrometry studies with single [63], and multiple penetration depths using interdigital electrodes have continued in several countries [64, 65]. Interdigital sensors are really popular for their one side access and their ability for non-destructive testing (NDT). Nondestructive testing or NDT is defined as the use of noninvasive techniques to determine the integrity of a material, component or structure or quantitatively measure some characteristics of an object. So in short NDT can be used to measure or read the properties of a material without physically altering it. NDT is applied in various industries during production, quality maintenance and also to check the durability of the product while in use. The use of NDT is more helpful in the severe hazardous environments.. 33.

(53) 3.3 Applications of Interdigital Sensors The sensitivity of interdigital sensor changes with a change in geometric parameters [66], even the variations within the production tolerances of a manufacturer changes the response of sensors greatly. Interdigital sensors are used to study and monitor the dielectric properties of the insulating materials as the dielectric property starts to cease with age and absorption of water [67]. The moisture content in pulp can be measured up to 96% using Interdigital electrodes whereas it is limited to only 90% using other methods [31], Interdigital electrodes offers single-sided measurements and high sensitivity unlike other methods and can be used in normal working conditions as well. The Interdigital sensors are used in Biomedical field to monitor the change in impedance caused by the growth of immobilized bacteria [68], when the sensor is immersed into a liquid the bacteria present in the solution gets hooked to the electrode there by causing a change in impedance. A Micro Sensor based on interdigital electrodes is used to measure the water content in the human body as the water content in the skin could be used as an index to confirm the health of human skin [69]. The Micro Sensor measurements are compared with standard skin moisture measuring instrument and their results matches at a specific frequency which is encouraging. Infrared Spectrometers are used to measure the fat to protein content of milk which is an important factor for processing of milk by dairy companies. Infrared Spectrometry is an expensive and a heavy system which cannot be carried around with ease [70], this can be substituted with an Interdigital sensor. The Sensor responds well to the fat concentration, the impedance decreases with the increase in fat content. The fat content in other dairy products such as butter, cheese, curd and yoghurt can also be determined using these sensors. As in the estimation of fat content of milk products, the Interdigital sensors can also be used for the estimation of fat content in pork meat [71]. For this, three sensors were designed and tests were done with different samples of pork meat at different orientations. Quadratic and Cubic expressions were calculated to determine the fat and protein content in each sample, and the experimental results came close to the values predicted using chemical analysis. The Interdigital sensors are used to inspect the quality of Saxophone reeds [72] the measured impedance of the sensor is used to predict the properties of each reed. The reeds involved in experiments were earlier used for playing saxophone for qualitative analysis to avoid bias. 34.

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